Current methods used for inspection of storage tanks require access to their internal surfaces and a visual assessment of corrosion. This is both a time consuming and expensive procedure, requiring emptying of the tanks and inhospitable working conditions. An alternative to local inspection is to use guided waves to inspect large areas from a sensor. This approach has been used for corrosion detection in oil pipelines, and more recently in the development of methods for inspection of large areas of plates; see P Wilcox, M Lowe and P Cawley, “An EMAT array for the rapid inspection of large structures using guided waves”, Review of progress in quantitative NDT, eds. D O Thompson and D E Chimenti, A.I.P. Conference Proceedings, Vol 22, 2003; P Wilcox, M Lowe and P Cawley, “Omni-directional Guided Wave Inspection of Large Metallic Plate Structures Using an EMAT Array”, IEEE Transactions on Ultrasonics Ferroelectrics and Frequency Control, 2004; P Wilcox, M J S Lowe and P Cawley, “The effect of dispersion on long range inspection using ultrasonic guided waves”, NDT & E International, 34, pp 1-9, 2001. The technique employed involves scanning a two dimensional plate in all directions by generating an electronically steered ultrasonic beam with an EMAT transducer of a special construction.
Ultrasonic waves employed in plates in non-destructive testing (NDT) applications are commonly Lamb waves. Lamb waves are defined and explained in Viktorov, Acoustic Institute, Academy of Sciences, USSR “Rayleigh and Lamb Waves”, Plenum, 1967: “Lamb waves refer to elastic perturbations propagating in a solid plate (or layer) with free boundaries, for which displacements occur both in the direction of wave propagation and perpendicularly to the plane of the plate. Lamb waves represent one of the types of normal or plate modes in an elastic waveguide, in this case a plate with free boundaries. For this reason Lamb waves are sometimes simply called normal modes in a plate. But this definition is rather loose, insofar as another type of normal mode can exist in a plate with free boundaries, namely transverse normal modes, wherein the motion is perpendicular to the direction of propagation and parallel to the boundaries of the plate.”
Analysis reveals that for wave motion perpendicular to the direction of propagation, and perpendicular to the thickness of the plate, there are two groups of waves, so-called symmetrical Lamb waves (S) and anti-symmetrical Lamb waves (A). In symmetrical waves, the motion resembles alternate expansion and contraction of the plate in the thickness direction, whereas in anti-symmetrical waves, the wave motion resembles alternate bending of the plate in opposite directions across the thickness of the plate. There exists a finite number of each type of wave, termed orders (n), with the wavelength of higher orders harmonically related to the zeroth order. The orders differ from one another by their phase and group velocities.
There are other types of wave that propagate in plates that are not usually termed Lamb Waves, for example shear (SH) or transverse waves wherein the wave motion is parallel to the plate boundaries.
Guided waves in plates will leak energy into the surrounding media if the plate is immersed in a liquid. This will be particularly marked if the media is viscous, and will occur much more where the mode type results in the surface deformation having out-of-plane motion, rather than in-plane motion. In general, this means antisymmetric Lamb wave modes have high attenuation, and symmetric Lamb Wave modes with little out-plane surface displacement will show insignificant attenuation as a result of liquid loading. However, this usually means that specialised transducers are required which are sensitive to in-plane surface displacement. Surface mounted EMATs (Electromagnetic acoustic transducers) and shear piezoelectric transducers can be used to generate and receive symmetric modes, such as the fundamental S0 or SHo modes.
Complications associated with guided wave inspection arise because, in general, the waves propagate with different velocities at different frequencies (a phenomenon known as dispersion), and also because the excitation and reception of unwanted modes result in overlapping and confusing signals. Typical practical problems associated with the use of guided waves also arise from reflections from plate edges, the presence of stiffeners, the presence of welds and joints, and thickness changes.